[entity/optimization] Cache rset when entity.related is called with entities=False
If fail to see why we couldn't cache in this case, while this is important for
optimization reason: when doing a single HTTP request, some queries are done two
or three times because of predicates / uicfg or alike.
Also always store tuple and not list in the cache, because:
* else we get some regression
* and inconsistent result type (tuple or list)
* and it simply feels better to cache an unmutable object.
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